Laser absorption provides a non-intrusive method to sense temperature, pressure, and species concentrations in gas phase systems. Wavelength-modulation spectroscopy (WMS) is a method for sensitively detecting laser absorption and is typically performed by monitoring the transmitted laser power at the second harmonic of the modulation frequency (WMS-2f). The use of injection-current tuned diode lasers provides a signal at the first harmonic (1f) of the modulation frequency that is proportional to laser power.
For decades, sensitive tunable diode laser (TDL) absorption measurements have been performed with wavelength modulation spectroscopy (WMS) in a wide variety of practical applications. With its better noise-rejection characteristics through laser wavelength modulation strategies, WMS has long been recognized as the method of choice for sensitive measurements of small values of absorption, and thus is favored for trace species detection, measurements with significant noise, or both. Injection current-tuned TDL-WMS is attractive in systems with significant non-absorption attenuation of the optical transmission because it is possible to account for changes in laser intensity by normalizing the signals at even or odd harmonics of the modulation by the first harmonic signal.
Most early TDL-WMS applications were either in an atmospheric pressure open path or in a relatively low-pressure (P<2 atm) vessel, where the optimal modulation depths for achieving the strongest WMS signal were small and accessible within the tuning range of typical early TDLs. However, to achieve optimal WMS conditions for higher-pressure gas sensing, a larger modulation depth is required because the collisional width of the targeted transition is broadened. Based on a previous model describing the WMS-nf signals by Fourier analysis, TDL-WMS measurements with 2f detection at pressures to 10 atm using a large modulation depth were demonstrated.
Here, the concept of 1f-normalized WMS-2f considering the non-ideal modulation characteristics of injection current-tuned TDLs was discussed. Later, others developed a general protocol for 1f-normalized TDL-WMS measurements. The normalization scheme makes the WMS measurements independent of the laser intensity, and thus allows a calibration-free WMS measurement without the need to acquire the zero-absorption baseline during the absorption measurements. This benefit is important, since at pressures >10 atm where the transitions are strongly blended, it is generally difficult to find a near zero-absorption region near the targeted transition for the measurement of a direct absorption (DA) baseline. In addition, laboratory bench measurements indicate that WMS measurements have smaller uncertainties than DA for non-Lorentzian lineshape effects at high pressures, such as the breakdown of the impact approximation and line-mixing.
To meet more rigorous criteria for environmental-unfriendly emissions and to increase the energy efficiency, in-situ sensors are needed to optimize the performance of next-generation energy systems. Tunable diode laser absorption spectroscopy (TDLAS) offers potential for in-situ, non-intrusive, fast sensors for monitoring gas composition, temperature, pressure and velocity. With the emergence of the mature, reliable, narrow line-width wavelength-tunable diode lasers in the past two decades, such absorption sensors transitioned from laboratory measurements to use in conditions at harsh industrial facilities. Two schemes for TDL absorption have emerged for practical sensors, scanned-wavelength direct absorption (scanned-DA) and wavelength modulation spectroscopy (WMS).
Scanned-DA is most often used for its simple interpretation of the measured absorption signals in terms of the gas properties of temperature and composition, especially for facilities where the gas pressure is relatively low and for species with well-resolved absorption transitions. However, a scanned-DA measurement requires determination of a zero-absorption baseline, which is difficult to attain for high pressure environments where the collisional broadening blends neighboring transitions and eliminates regions of zero-absorption for the scan region of the laser. This can be even more challenging if the non-absorption transmission loss is time-varying, i.e. caused by beam scattering by coal particles in an entrained-flow coal gasifier, by bed particles in fluidized bed reactors, by the fly ash in a power plant economizer exit or in a waste incinerator. WMS using injection current-tuned diode lasers offers the potential of normalization of the 2f absorption signal by the 1f signal to account for time-varying non-absorption losses. This is possible because the signals for all the WMS harmonics are proportional to the laser power and the laser intensity modulation accompanying the injection-current modulation dominates the 1f signal for optically-thin conditions. This normalization enables quantitative WMS absorption measurements without determining a zero-absorption baseline. In addition, for WMS measurements performed with a modulation frequency larger than a few kHz, the detected harmonics can be isolated from the low-frequency noise. The bandwidth of the WMS measurements can be adjusted by the lowpass filter to balance the noise and the desired measurement time resolution. These are some of the features that make WMS an attractive alternative to DA for absorption measurements in harsh environments.
Although WMS has advantages over DA in noise-rejection and does not require knowledge about the zero-absorption baseline, previous calibration-free WMS measurements require laboratory characterization of the laser tuning and an accurate spectral model including collisional broadening to interpret absolute gas properties from the measured WMS signals. Although there is a large literature of models to simulate WMS spectra, it is common for a typical WMS model to involve many mathematical expressions, which can make the WMS technique complicated. The model becomes even more complex when the analytical WMS signals are explicitly expressed for an explicit lineshape function. Thus, almost all models have been simplified by assumptions. For example, some models can only be used for conditions where the intensity modulation is not important, the modulation depth is small, or the modulation frequency is low. Others are only accurate when the intensity modulation is linear, and may not be suitable for external cavity lasers where the non-linearity in intensity modulation can be large, or in a system where the optical components are wavelength dependent, such as a semiconductor optical amplifier or interference in the transmission produced by etalons from optical components with parallel surfaces. These complexities and limitations are even more pronounced for scanned-wavelength modulation spectroscopy where the laser-dynamics cannot be accurately described by a Fourier series of only one modulation frequency.
Most calibration-free WMS methods require an accurate estimation of the collisional broadening database, involving a heavy workload to pre-measure the broadening coefficients of the selected transitions at a range of temperatures. The total lineshape is then estimated based upon the measured coefficients of each broadening partner, and the gas composition in the target application. This estimation can have large uncertainties when the gas composition cannot be accurately known.
What is needed is a unified, accurate approach to determine the WMS absorption lineshape spectra at multiple harmonics of the modulation frequency for providing a sensor that is appropriate for high-pressure, particulate laden environments, e.g., in order to allow control of commercial coal gasification, where a need for a non-absorption baseline measurement is removed from measurements in environments where collision broadening has blended transition linewidths, where calibration free WMS measurements without knowledge of the transition linewidth is enabled.